ABSTRACT Natural Killer (NK) cells play important roles in anti-viral and anti-tumoral immune responses, eliminating abnor- mal cells by direct lysis and/or by cytokine-mediated immunoregulation, and they are increasingly recognized as promising cellular tools for immunotherapy. Caspases are evolutionarily conserved proteases that play central roles in cell death induction. The role of certain caspases in immune responses has been extensively studied, but the immune functions of others remain largely understudied, particularly the pro-apoptotic caspase-3 (Casp3), Casp7 and Casp9. There is a need to correct this knowledge gap, to reveal opportunities for pharma- cological modulation of immune responses, using existing drugs that target the enzymatic activity of caspases. Our long-term goal is to more fully elucidate the molecular mechanisms by which apoptotic caspases participate in immune regulation. Using mice that we recently developed to lack Casp9 or both Casp3/7 in their immune systems, we now demonstrate that these caspases are required for the maturation and function of NK cells. Indeed, caspase-deficient mice present severe defects in NK cells numbers, maturation and function. The overall objective of the proposed project is to elucidate the cellular mechanisms underlying our observations to date. Our central hypothesis, guided by our preliminary data demonstrating the cell extrinsic nature of this phenotype, is that caspase-deficiency in an unidentified cell type is responsible for the observed NK cell defects. We will test our hypothesis following two specific aims: (1) we will characterize the acquired NK cell defects in the absence of caspases, including proliferation, survival and functional properties, in mouse and humanized mouse models; and (2) we will identify the cellular source of the non-autonomous, caspase-regulated factor that inhibits NK cells. The rationale for the proposed research is that NK cells are essential players in disease-fighting innate immune responses, and a fundamental understanding of their biology has the potential to facilitate the future development of strategies to therapeutically harness NK cells. The predicted outcome of our work is that we will reveal the cellular basis of a previously unrecognized, mechanism of apoptotic caspase-mediated regulation of NK cell biology.